Industrial thermal processes involving a cooling substance called the “refrigerant” that transfers heat to a heat-absorbing substance, called the “absorbent,” have been utilized since the 18th century. Various published patent documents, including U.S. Pat. Nos. 8,490,427, 7,938,888, 6,463,750, 4,819,446, 4,205,529, 4,227,375, and 4,373,347, describe using desiccant in either liquid or solid forms for the absorption of the vapor state of a liquid or solid substance that has a substantial vapor pressure. The vapor absorption encourages the refrigerant to evaporate further, thus generating a cooling of the refrigerant. The desiccant's absorption of vapor is generally exothermic, which has the effect of heating the desiccant. This effect can be used to generate a cooling effect, a heating effect, or both.
By 1810, a process utilizing sulfuric acid to cool water had been developed. This process worked by absorbing water vapor contained in a closed evacuated chamber when both water and sulfuric acid were separately contained within the same chamber. This conventional process results in evaporation of vapor from a pool of water, eventually generating ice at the surface of the water. By 1859, water and ammonia were utilized together for a similar effect, and by 1860, this thermal transfer process was patented. This classical invention for thermal transfer from the 19th century enabled the development of an ice-making industry and refrigerated food storage in modern civilization.
During the 1950's, lithium bromide (LiBr) was introduced in the absorption refrigeration industry, as it had several advantages over the ammonia/water cycle. Advantages included non-volatility of LiBr and the ability to pump the solution with liquid pumps rather than gas pumps. By 1956, double stage LiBr-based absorption refrigerators had been developed. Today, multiple-stage absorption refrigerators have been developed by utilizing the same design principle used in the double stage design from the 1950's.
In all conventional cases described above, a low temperature liquid or solid refrigerant generates a vapor, which is absorbed by a desiccant at another temperature. Several conventional devices may be able to utilize the cool refrigerant in the absorption of heat from another source, such as a stream of air passing through a room, duct, or other enclosed area. This is used for air conditioning or refrigeration. Once the desiccant is at least partially saturated with absorbed refrigerant, the desiccant generally becomes unusable. As a result, the desiccant must be regenerated, or separated from the refrigerant. As all conventional refrigeration devices utilize a closed system, they necessitate a heat transfer into the system, which drives the refrigerant from the desiccant. The heat carried away with the refrigerant is then rejected, enabling the refrigerant to recondense and be used again.
At least one device described in U.S. Pat. No. 4,227,375 utilizes the heat generated from the absorption of vapor as a method of obtaining thermal energy. This is a form of chemical energy obtained by the adsorption of water by the desiccant. Storage of the dry desiccant enables the device described by U.S. Pat. No. 4,227,375 to effectively store energy in a convenient and stable form.
An important aspect of the absorption-style refrigeration is that the cooling obtained is a non-equilibrium process. The cooling is obtained when the vaporized refrigerant is absorbed, thus generating vaporization of more liquid or solid refrigerant. In absorption refrigerators, the latent and enthalpy-associated heat is rejected to an intermediate-temperature reservoir, which enables a continual absorption of heat and vapor by the desiccant. As a result, the system is typically operated out of chemical and thermodynamic equilibrium. This ensures rapid heat transfer and maximal cooling.
The need to simultaneously keep the desiccant active and to operate at low temperatures require the cool and concentrated desiccant to be continually pumped into the absorption chamber in practice. Furthermore, it is also necessary to continually pump the warmer dilute desiccant into the regenerator. Because the desiccant is a very hygroscopic substance, a much higher temperature heat than the intermediate temperature (i.e. at which the heat is rejected) must be utilized to vaporize the refrigerant either absorbed or adsorbed by the desiccant. Therefore, the absorption refrigeration system typically includes at least three different temperature regimes.
In addition, as mentioned previously, there are absorption refrigerators that utilize multiple stages. One such device is disclosed in U.S. Pat. No. 8,783,053. These multi-stage devices utilize high temperature heat sources, much like single stage devices. The heat rejected from the regenerator of the first stage is used to drive a second stage regenerator. This results in a lower thermal input and a higher coefficient of performance (COP) value. However, the addition of the second stage or any subsequent stages increases the system complexity, as the multiple stages need to be operated at differing operational pressures.
The conventional multiple stage designs are used to drive the desiccant used in the absorption to a continually lower water content, thereby enabling both quicker thermal transfers and more effective cooling. However, the conventional multiple stage designs are not able to increase the overall thermal gradient between the desiccant and the refrigerant as a linear or superlinear function of the number of stages. Therefore, the maximal thermal gradient is still limited to that generated between a completely dry desiccant or desiccant solution and the refrigerant in a completely closed container filled only with vaporized refrigerant.
In 2010, the inventor of the present invention, Dr. Sanza T. Kazadi, and other researchers disclosed a design for a cooling device based on an innovative entrochemical cell. The entrochemical cell has some similar physical characteristics to the absorption refrigerator. Each entrochemical comprises two closed chambers in vapor communication with one-another, and are arranged in a manner to ensure that the combined system is hermetically sealed in operation. Preferably, one chamber contains a desiccant while the other chamber contains a refrigerant.
However, unlike the absorption refrigerator, the entrochemical cell is operated at or near chemical and thermodynamic equilibrium. The two solutions contained by the entrochemical cell can only obtain chemical equilibrium when their temperatures are different. As a result, the two chambers settle to different temperatures. In an efficiently-insulated entrochemical cell, the temperature gradient may be maintained for days or weeks.
The table below demonstrates equilibrium thermal gradients in an entrochemical cell when one cell contains distilled water and the other contains a saturated solution of the indicated salt or salt combination.
SoluteΔT ° C. (±0.5° C.)NaCl3MgSO41NaCl + MgSO44KNO31.5KNO3 + NaCl4NaNO35KCl1.5NaNO3 + KCl7
In the table above, the thermal gradient between an entrochemical cell's chambers at equilibrium is shown, when a first chamber is filled with distilled water and a second chamber is filled with the indicated salt or salt combination.
These experiments, documented by Kazadi et. al., involved placing a water/salt solution in the first chamber and distilled water in the second chamber, and then measuring the temperature after the system comes to equilibrium. It was demonstrated that, when the entrochemical cell was evacuated in order to generate a rarified atmosphere near the vapor pressure of the liquid inside, the two water solutions generated a thermal gradient that could be sustained for hours to weeks if properly insulated. Moreover, if the resulting dilute salt solutions are removed from the chambers, they can be dried in air and reused. While this last property may be quite appealing in some cases, as it eliminates the need for a regenerator using high temperature heat, the thermal lift of the various solutions remains undesirably low.
When operated at or near chemical equilibrium, a single absorption cell responds to perturbations or thermal leakage, which tends to bring the refrigerant and desiccant temperatures closer to one-another, by evaporation of the refrigerant and absorption of vapor by the desiccant. The evaporation process can be quite energetic, generating winds with speed of several miles per hour inside the cell, depending on the particular geometry of the cell. As a result of this evaporation, the amount of leakage or perturbation a cell can effectively respond to is limited. The refrigerant is eventually exhausted and must be replaced. In a closed system, this is accomplished using a regenerator as with conventional systems. In an open system, this is accomplished by simply adding more refrigerant.
Much of the present research on absorption refrigerators is focused on generating new chemical compounds that can function in the refrigerator while avoiding problems such as crystallization, clogging, or corroding the mechanical parts of the device, etc. This is necessary because of the dual problem of finding a desiccant that can be regenerated at low temperature and can simultaneously enable a large thermal gradient. However, if the design enabled the individual cells to reject heat into one another to make the warmer part of one cell in its chemical equilibrium state with differing chamber temperatures identical in temperature to the colder chamber of the next cell, the overall thermal gradient may be increased dramatically. Conventional absorption refrigerator have not been able to achieve such desirable characteristics. Therefore, an innovative design that enables the use of much less chemically active desiccants that are commonly available and inexpensive, such as NaCl, may be highly desirable.
Kazadi et. al. has previously disclosed a two-stage device in which a NaCl plus water solution was used in a first chamber and a distilled water solution was used in a second chamber. While this design seemed to provide superlinear performance with a one-chamber thermal lift of 2.5° C. and a two-chamber thermal lift of 5.7° C., the design could not be extended to three stages. Moreover the design did not employ the nested heat transfer system of the present invention.
When the refrigerant condenses in or on the desiccant, the desiccant surface becomes covered with water. This can have the effect of shielding the interior of the desiccant from additional water absorption or adsorption. As a result, when working with pooled liquid desiccants, it is necessary to mix the desiccant to restore the desiccant concentration at the surface of the water. This can be accomplished by a variety of means, including adding a physical mixer, sloshing the cell, or utilizing the thermal energy in the vapor flow between the two chambers to drive a mechanical mixing system. This functionality has not been previously disclosed due to the vastly different nature of conventional absorption refrigerator design.
Therefore, it may be desirable to devise an absorption refrigeration device which operates in chemical equilibrium and can additively combine the thermal lift of multiple cells with independent desiccant and refrigerant equilibria, enabling the use of many different desiccants with limited thermal gradients and associated thermodynamic properties.
In conventional LiBr type absorption refrigerators, both the desiccant and the refrigerant are sprayed into a chamber using atomizers or similar systems for creating very small droplets. This increases the surface area, enabling quicker absorption of water vapor by the desiccant and evaporation of water from the refrigerant droplets. However, this requires the use of at least two fluid pumps and two atomizers. These additional elements add to the complexity of the absorption refrigerator. Therefore, it may also be desirable to devise a novel device that reduces design complexity exhibited in conventional absorption refrigerator designs, while providing a good desorption and absorption efficiency.
Furthermore, it may also be desirable to provide a novel device that utilizes an absorption and desorption process between a desiccant and a refrigerant that functions to establish, maintain, and return to a chemical equilibrium state, consequently creating and maintaining a thermal gradient. In addition, it may also be desirable to provide a novel device that allows a plurality of individual cells to perform their absorption and desorption independently to achieve an overall thermal gradient greater than that of an absorption and desorption reaction within a single cell, thereby enabling an expansion of the range of desiccants available for absorption refrigeration and related thermal processes.
Moreover, it may also be desirable to devise a system that accommodates an additive use of a desiccant for achieving a substantially larger thermal gradient than that of a single stage device, and does not require water pumps, atomizers, or other tools to provide sufficient interaction between the water vapor and either the desiccant or the refrigerant. Furthermore, it may be desirable to provide a mixing method required to achieve a high performance for the entrochemical cell. In addition, it may also be desirable to increase the surface area interface between the vapor and the desiccant.